Maybe my advice is not relevant because I do not know which is your level in mathematics

a, b and c being solutions of with

if you study the variations of the function by finding the derivative, you will see that f :
is increasing from 0 to 1 with f(0) = -abc < 0 and f(1) = 4 - abc which must be > 0
is decreasing between 1 and 3 with f(1) = 4 - abc > 0 and f(3) = -abc < 0
in increasing between 3 and 4 with f(3) = -abc < 0 and f(4) = 4-abc > 0

Therefore one of the solutions is between 0 and 1, one is between 1 and 3 and one is between 3 and 4